1. Field of the Invention
This invention generally relates to a class of endoprostheses known as "stents" and more specifically to the structure and manufacture of such stents and the assembly of such stents into delivery systems.
2. Description of Related Art
Certain medical devices, called "stents", are well known and have a variety of forms. For example, U.S. Pat. No. 4,690,684 of Sep. 1, 1987 to McGreevy et al for a "Meltable Stent for Anastomosis" discloses a solid stent formed of a biologically compatible material, such a frozen blood plasma or the like. According to the disclosure, a solid stent of this type may be inserted into opposed ends of a ruptured vessel to support the separated vessel walls while the ends are bonded together. The heat from the bonding operation and the body eventually melt the stent and clear the vessel.
A stent that constitutes an endoprosthesis usually comprises a tubular structure that expands radially to be implanted into the tissue surrounding a "vessel" thereby to maintain its patency. It is well known that stents may be utilized in body canals, blood vessels, ducts and other body passageways, and the term "vessel" is meant to include all such passageways. Generally speaking, a stent delivery system includes the stent and some means for positioning and fixing the stent in place. Typically, the stent delivery system includes a catheter that supports the stent in a compacted form for transport to a site of implantation. Means integral with or ancillary to the catheter then expand the stent radially into the vessel walls to be implanted at the selected site. After the catheter is removed, the stent retains an expanded shape to keep the vessel walls from closing.
Stent delivery systems must conform to several important criteria. First, it is important to keep the transverse dimension of the delivery system to a minimum, so the stent must be capable of compaction against a delivery device, such as a catheter. Second, the delivery system must facilitate the deployment of the stent into contact with the vessel walls once it is located in a body. Third, the stent delivery system must easily disengage from the stent after the stent is deployed. Fourth, the procedure for removing the delivery system from the body must be straightforward. Fifth, the delivery system must operate reliably.
U.S. Pat. No. 4,922,905 of Ernst P. Strecker for a "Dilatation Catheter" describes the manufacture, construction and use of such stents and is incorporated herein by reference. In the specific disclosure of the Strecker patent, the stent comprises a tubular structure that is knitted from metal or plastic filament in loosely interlocked loops. A stent delivery system from metal or includes a balloon catheter and a coaxial sheath. The balloon catheter supports the compacted stent during its transport to a site within the body. The sheath covers the stent to prevent premature stent expansion and to facilitate the transfer of the stent through various passages in the body. A physician properly locates the stent, and then moves the sheath axially with respect to the catheter thereby to expose the stent. Then the physician operates a balloon pumping system to expand the balloon catheter and move the stent into a final configuration in contact with tissue surrounding the stent. When the stent expands radially, the filament material undergoes a plastic deformation. Consequently, the stent retains its new expanded shape. When the balloon subsequently deflates, it is free of the expanded stent, so the catheter, sheath and remainder of the delivery system can be withdrawn from the patient.
Commercial embodiments of the structures shown in the Strecker patent include rings for overlapping the end portions of the compacted stent thereby to eliminate the sheath. In such embodiments, however, the entire assembly of the catheter and compacted stent slides into position after passing through a previously positioned introducer sheath.
U.S. Pat. No. 4,733,665 of Mar. 29, 1988 to Palmaz for an "Expandable Intraluminal Graft, and Method and Apparatus for Implanting an Expandable Interluminal Graft" discloses a catheter with rings for positioning a compacted stent on a balloon portion of the catheter. A sleeve encases the compact stent. When the stent is properly positioned, a physician retracts the sleeve and pumps the catheter to expand the stent into position. During its expansion the stent detaches from the mounting rings. Then the physician deflates the balloon and removes the catheter, leaving the stent in place.
Other patents disclose other devices and operators for releasing stents. For example, in some stents the compaction process introduces stresses into the stent materials that act to expand the stent after its release from a sleeve or similar restraint. The following patents disclose examples of such structures:
U.S. Pat. No. 4,580,568 (1986) Gianturco PA1 U.S. Pat. No. 4,665,918 (1987) Garza et al PA1 U.S. Pat. No. 4,913,141 (1990) Hillstead PA1 U.S. Pat. No. 3,868,956 (1975) Alfidi et al PA1 U.S. Pat. No. 4,512,338 (1985) Balko et al PA1 U.S. Pat. No. 4,799,479 (1989) Spears
Other patents disclose various structures in which heat expands the stent and include:
U.S. Pat. No. 5,026,377 of Jun. 25, 1991 to Burton et al for a "Stent Placement Instrument and Method" discloses a delivery system for a self-expanding stent. The stent is a braided structure formed of a shape memory material. An outer sleeve retains the stent radially during transport to a final site within the body. A grip member enables both deployment and retraction of the stent. There are several examples of grip members in this patent. One, for example, comprises a releasable adhesive on a support for the stent. The adhesive grips the stent without slipping while the stent is in the instrument, but allows the stent to expand when a outer sleeve is retracted.
As known the overall diameter and flexibility of a stent and its delivery system determine the range of vessels that can receive a stent. It is important that any stent structure have as small an overall diameter as possible. The smaller the diameter, the greater the range of vessels for which the endoprosthesis becomes viable. That range of vessels is limited with prior art structures particularly by a protective sheath or the like that surrounds a stent and has two functions. First, the protective sheath provides a smooth surface over the stent to facilitate its transport through the body with minimal trauma. Second, the protective sheath prevents the stent from expanding prematurely. The second function determines the wall thickness of a sheath or like structure and with it the overall diameter of the stent delivery system. The wall must be sufficiently thick to provide the strength necessary to restrain the stent. This thickness is greater than the wall thickness required by the first function. For a given diameter stent, the overall diameter of the stent and the sheath or the like can exceed a minimal diameter. It is this characteristic that prevents the introduction of prior art stents into smaller vessels.